The present invention relates to a method for fabricating a semiconductor device; and, more particularly, to a method for fabricating a semiconductor device suitable for an image sensor having microlenses.
Recently, most of communications apparatuses are required to process not only voice signals but also image is signals and to that purpose, a color image sensor is generally employed.
The color image sensor usually includes a multiplicity of pixels, each having a metal pattern, color filter elements, and microlenses. The metal pattern has cell electrodes and bonding pads. The color filter elements are formed on a dielectric layer overlying the cell electrodes. The microlenses are positioned over the color filter elements and act to focus incident light onto the color filter elements.
Since a semiconductor device suitable for the image sensor used to be priced high, a fabrication yield thereof could be somewhat disregarded hitherto. However, a recent oversupply of semiconductor devices reduces the high value-added advantage of the image sensor and therefore the fabrication yield becomes an important factor to be seriously taken into account. Whether each of the microlenses has a desired radius of curvature is one of the major factors for achieving a high fabrication yield of the image sensor.
FIGS. 1A to 1I are sectional views illustrating a sequence of conventional processes for fabricating a semiconductor device suitable for an image sensor.
In FIG. 1A, a lower insulating layer 4 is formed on a substrate 2 where a photoelectric conversion element (not shown), such as a charge coupled device (CCD) or a photodiode, and logic circuits (not shown) are arranged. In FIG. 1B, after a metallic material is formed on the lower insulating layer 4, a photolithography is applied thereto, so that a metallic pattern including a bonding pad 6 is formed thereon. Herein, some portions of the lower insulating layer 4 are selectively exposed through the metallic pattern.
In FIG. 1C, a planar upper insulating layer 8 is formed, e.g., by means of a deposition, on the bonding pad 6 and the substrate 2. In FIG. 1D, a first photosensitive material is coated on the upper insulating layer 8 and then selectively removed, so that at least one color filter element 10 is formed thereon. The coating and removing of another first photosensitive material is repeated until a multiplicity of color filter elements 10, e.g., three or four different types, are formed thereon. If three types of color filter elements are present, they may represent red, green, and blue, respectively.
In FIG. 1E, a second photosensitive material is coated on the upper insulating layer 8 and then selectively removed, so that a planar layer 12 is formed to cover the color filter elements 10. In FIG. 1F, a third photosensitive material is coated over the substrate 2 and then selectively removed, so that the third photosensitive material remains on the planar layer 12 only to overlie the color filter elements 10. The third photosensitive material is subsequently heat-treated to form microlenses 14, each of which is desired to have an equal radius of curvature. Each microlens 14 corresponds to one of the color filter elements 10.
In FIG. 1G, a thin oxide layer 16 is formed over the substrate 2 where the microlenses 14 are shaped. Then, as shown in FIG. 1H, a photolithography is applied thereto, so that some portions of the oxide layer 16 and the upper insulating layer 8 are removed to uncover a corresponding portion of the bonding pad 6. The photolithography is the process of transferring patterns of geometric shapes on a mask to a thin layer of photosensitive material called photoresist. Generally, the photolithography includes the steps of coating, exposing, developing, baking, and etching.
The upper insulating layer 8 protects the bonding pad 6 during the process of forming the microlenses 14 or the color filter elements 10. If the bonding pad 6 is exposed to the above-mentioned process, it may be degraded. The degradation of the bonding pad 6 can result in yield loss due to poor wire bonds, or reliability failures in terms of wire bond failures occurring during the lifetime of the image sensor.
While selectively removing the upper insulating layer 8 during the photolithography process, the photosensitive elements such as the microlenses 14, the planar layer 12, and the color filter elements 10 should be protected and the oxide layer 16 is employed for the purpose. The process of forming the oxide layer 16, however, gives rise to a drawback in that the color filter elements 10 or the microlenses 14 may be deformed to thereby reduce the fabrication yield. FIG. 1I shows the microlenses 14 deformed in the process of forming the oxide layer 16.
The color filter elements 10 and the microlenses 14 made of the photosensitive material are usually deformed at a higher temperature than about 200xc2x0 C. Therefore, to prevent the deformation thereof, the oxide layer 16 should be formed at a lower temperature than about 200xc2x0 C. At such a relatively low temperature, however, the thickness of the oxide layer 16 tends to become irregular and it is very difficult to minimize the amount of particles produced in the process for forming the oxide layer 16. These drawbacks result in a low yield of the conventional method for fabricating a semiconductor device suitable for an image sensor.
It is, therefore, an object of the present invention to provide a method for fabricating a semiconductor device suitable for an image sensor, wherein degradation of a bonding pad and deformation of microlenses are avoided during the fabrication process.
In accordance with a preferred embodiment of the invention, there is provided a method for fabricating a semiconductor device suitable for an image sensor, the method including the steps of: forming a lower insulating layer on a substrate; forming a bonding pad on the lower insulating layer; forming an upper insulating layer on the lower insulating layer to cover the bonding pad; selectively removing the upper insulating layer to uncover a top portion of the bonding pad; forming a protection layer on the-upper insulating layer to shield the uncovered portion of the bonding pad; forming a multiplicity of color filter elements on the protection layer; forming a planar layer to cover the color filter elements; and forming a multiplicity of microlenses on the planar layer.
In accordance with another preferred embodiment of the present invention, there is provided a fabricating method for a semiconductor device suitable for an image sensor, the method including the steps of: forming a lower insulating layer on a substrate; forming a bonding pad on the lower insulating layer; forming an upper insulating layer on the lower insulating layer to cover the bonding pad; selectively removing the upper insulating layer to reduce a thickness thereof on the bonding pad; forming a multiplicity of color filter elements on the upper insulating layer; forming a planar layer to cover the color filter elements; selectively removing the upper insulating layer with the reduced thickness to uncover the bonding pad; and forming a multiplicity of microlenses on the planar layer.
In accordance with still another preferred embodiment of the present invention, there is provided a manufacturing method for a semiconductor device suitable for an image sensor, the method including the steps of: forming a lower insulating layer on a substrate; forming a bonding pad on the lower insulating layer; forming an upper insulating layer on the lower insulating layer to cover the bonding pad; selectively removing the upper insulating layer to uncover a top portion of the bonding pad; oxidizing the uncovered portion of the bonding pad to form a metallic oxide layer thereon; forming a multiplicity of color filter elements on the upper insulating layer; forming a planar layer to cover the color filter elements; and forming a multiplicity of microlenses on the planar layer.